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==Universo==
===Philosophical models===
{{Vedi anche|Cosmology}}
{{Vedi anche|Pre-Socratic philosophy|Physics (Aristotle)|Hindu cosmology|Islamic cosmology|Time}}
Dal sesto secolo aC, i filosofi presocratici greci svilupparono presto i modelli noti filosofico dell'Universo. I primi filosofi greci osservato che le apparenze possono ingannare, e cercato di comprendere la realtà sottostante dietro le apparenze. In particolare, hanno notato la capacità della materia di cambiare le forme (ad esempio, il ghiaccio di acqua a vapore) e filosofi hanno proposto che tutti i materiali apparentemente diversi paesi del mondo (legno, metallo, ecc) sono tutte forme diverse di un unico materiale , l'Arche. Il primo a farlo fu Talete, che ha chiamato questo materiale acqua. Seguendo lui, Anassimene lo chiamava Air, e postulato che ci deve essere attrattive e repulsive forze che causano l'arche di condensare o dissociarsi in forme diverse. Empedocle ha proposto che i materiali più fondamentali sono stati necessari per spiegare la diversità dell'universo, e ha proposto che tutti e quattro elementi classici (Terra, Aria, Fuoco e Acqua) esistito, anche se in diverse combinazioni e forme. Questa teoria quattro elementi è stato adottato da molti dei filosofi successivi. Alcuni filosofi prima Empedocle sostenuto cose meno materiali di arche; Eraclito sosteneva per un Logos, Pitagora credeva che tutte le cose erano composte da numeri, mentre Talete studente ', Anassimandro, ha proposto che tutto era composto da una sostanza nota come caotico apeiron, grosso modo corrispondente al concetto moderno di una schiuma quantistica. varie modifiche della teoria Apeiron sono stati proposti, in particolare quella di Anassagora, che ha proposto che la questione vari del mondo è stata scorporata da un apeiron in rapida rotazione, messo in moto dal principio di Nous (Mente). Ancora altri filosofi - in particolare Leucippo e Democrito - ha proposto che l'Universo era composto da atomi indivisibili si muove nello spazio vuoto, un vuoto; Aristotele opposti a questa visione ("La natura ha orrore del vuoto") per il fatto che la resistenza agli aumenti di movimento con densità; quindi, lo spazio vuoto dovrebbe offrire alcuna resistenza al moto, che porta alla possibilità di velocità infinita.
From the 6th century BCE, the [[pre-Socratic philosophy|pre-Socratic Greek philosophers]] developed the earliest known philosophical models of the Universe. The earliest Greek philosophers noted that appearances can be deceiving, and sought to understand the underlying reality behind the appearances. In particular, they noted the ability of matter to change forms (e.g., ice to water to steam) and several philosophers proposed that all the apparently different materials of the world (wood, metal, etc.) are all different forms of a single material, the [[arche]]. The first to do so was [[Thales]], who called this material [[Water (classical element)|Water]]. Following him, [[Anaximenes]] called it [[Air (classical element)|Air]], and posited that there must be attractive and repulsive [[force]]s that cause the arche to condense or dissociate into different forms. [[Empedocles]] proposed that multiple fundamental materials were necessary to explain the diversity of the universe, and proposed that all four classical elements (Earth, Air, Fire and Water) existed, albeit in different combinations and forms. This four-element theory was adopted by many of the subsequent philosophers. Some philosophers before Empedocles advocated less material things for the arche; [[Heraclitus]] argued for a [[Logos]], [[Pythagoras]] believed that all things were composed of [[number]]s, whereas Thales' student, [[Anaximander]], proposed that everything was composed of a chaotic substance known as [[Apeiron (cosmology)|apeiron]], roughly corresponding to the modern concept of a [[quantum foam]]. Various modifications of the apeiron theory were proposed, most notably that of [[Anaxagoras]], which proposed that the various matter in the world was spun off from a rapidly rotating apeiron, set in motion by the principle of [[Nous]] (Mind). Still other philosophers — most notably [[Leucippus]] and Democritus — proposed that the Universe was composed of indivisible [[atom]]s moving through empty space, a [[vacuum]]; [[Aristotle]] opposed this view ("Nature abhors a vacuum") on the grounds that [[Drag (physics)|resistance to motion]] increases with [[density]]; hence, empty space should offer no resistance to motion, leading to the possibility of infinite [[speed]].
Although Heraclitus argued for eternal change, his quasi-contemporary [[Parmenides]] made the radical suggestion that all change is an illusion, that the true underlying reality is eternally unchanging and of a single nature. Parmenides denoted this reality as το εν (The One). Parmenides' theory seemed implausible to many Greeks, but his student [[Zeno of Elea]] challenged them with several famous [[Zeno's paradoxes|paradoxes]]. Aristotle resolved these paradoxes by developing the notion of an infinitely divisible continuum, and applying it to [[space]] and [[time]].
The [[Indian philosophy|Indian philosopher]] [[Kanada]], founder of the [[Vaisheshika]] school, developed a theory of [[atomism]] and proposed that [[light]] and [[heat]] were varieties of the same substance.<ref>[[Will Durant]], ''Our Oriental Heritage'':
{{quote|"Two systems of Hindu thought propound physical theories suggestively similar to those of [[Ancient Greece|Greece]]. Kanada, founder of the Vaisheshika philosophy, held that the world was composed of atoms as many in kind as the various elements. The [[Jainism|Jains]] more nearly approximated to [[Democritus]] by teaching that all atoms were of the same kind, producing different effects by diverse modes of combinations. Kanada believed light and heat to be varieties of the same substance; [[Udayana]] taught that all heat comes from the sun; and [[Vācaspati Miśra|Vachaspati]], like Newton, interpreted light as composed of minute particles emitted by substances and striking the eye."}}</ref> In the 5th century AD, the [[Buddhist atomism|Buddhist atomist]] philosopher [[Dignāga]] proposed [[atom]]s to be point-sized, durationless, and made of energy. They denied the existence of substantial matter and proposed that movement consisted of momentary flashes of a stream of energy.<ref>F. Th. Stcherbatsky (1930, 1962), ''Buddhist Logic'', Volume 1, p.19, Dover, New York:
{{quote|"The Buddhists denied the existence of substantial matter altogether. Movement consists for them of moments, it is a staccato movement, momentary flashes of a stream of energy... "Everything is evanescent“,... says the Buddhist, because there is no stuff... Both systems <nowiki>[</nowiki>[[Samkhya|Sānkhya]], and later Indian Buddhism] share in common a tendency to push the analysis of Existence up to its minutest, last elements which are imagined as absolute qualities, or things possessing only one unique quality. They are called “qualities” (guna-dharma) in both systems in the sense of absolute qualities, a kind of atomic, or intra-atomic, energies of which the empirical things are composed. Both systems, therefore, agree in denying the objective reality of the categories of Substance and Quality,... and of the relation of Inference uniting them. There is in Sānkhya philosophy no separate existence of qualities. What we call quality is but a particular manifestation of a subtle entity. To every new unit of quality corresponds a subtle quantum of matter which is called guna “quality”, but represents a subtle substantive entity. The same applies to early Buddhism where all qualities are substantive... or, more precisely, dynamic entities, although they are also called dharmas ('qualities')."}}</ref>
The theory of [[temporal finitism]] was inspired by the doctrine of creation shared by the three [[Abrahamic religions]]: [[Judaism]], [[Christianity]] and [[Islam]]. The [[Christian philosophy|Christian philosopher]], [[John Philoponus]], presented the philosophical arguments against the ancient Greek notion of an infinite past. Philoponus' arguments against an infinite past were used by the [[Early Islamic philosophy|early Muslim philosopher]], [[Al-Kindi]] (Alkindus); the [[Jewish philosophy|Jewish philosopher]], [[Saadia Gaon]] (Saadia ben Joseph); and the [[Kalam|Muslim theologian]], [[Al-Ghazali]] (Algazel). They employed two logical arguments against an infinite past, the first being the "argument from the impossibility of the existence of an actual infinite", which states:<ref name=Craig>{{Cite journal|title=Whitrow and Popper on the Impossibility of an Infinite Past|first=William Lane|last=Craig|journal=The British Journal for the Philosophy of Science|volume=30|issue=2|date=June 1979|pages=165–170 [165–6]|doi=10.1093/bjps/30.2.165}}</ref>
:"An actual infinite cannot exist."
:"An infinite temporal regress of events is an actual infinite."
:"<math>\therefore</math> An infinite temporal regress of events cannot exist."
The second argument, the "argument from the impossibility of completing an actual infinite by successive addition", states:<ref name=Craig/>
:"An actual infinite cannot be completed by successive addition."
:"The temporal series of past events has been completed by successive addition."
:"<math>\therefore</math> The temporal series of past events cannot be an actual infinite."
Both arguments were adopted by later Christian philosophers and theologians, and the second argument in particular became more famous after it was adopted by [[Immanuel Kant]] in his thesis of the first [[antinomy]] concerning [[time]].<ref name=Craig/>
===Astronomical models===
{{Main|History of astronomy}}
Astronomical models of the Universe were proposed soon after [[astronomy]] began with the [[Babylonian astronomy|Babylonian astronomers]], who viewed the Universe as a [[Flat Earth|flat disk]] floating in the ocean, and this forms the premise for early Greek maps like those of [[Anaximander]] and [[Hecataeus of Miletus]].
Later [[Ancient Greece|Greek]] philosophers, observing the motions of the heavenly bodies, were concerned with developing models of the Universe based more profoundly on empirical evidence. The first coherent model was proposed by [[Eudoxus of Cnidos]]. According to this model, space and time are infinite and eternal, the Earth is spherical and stationary, and all other matter is confined to rotating concentric spheres. This model was refined by [[Callippus]] and [[Aristotle]], and brought into nearly perfect agreement with astronomical observations by [[Ptolemy]]. The success of this model is largely due to the mathematical fact that any function (such as the position of a planet) can be decomposed into a set of circular functions (the [[Fourier modes]]). However, not all Greek scientists accepted the geocentric model of the Universe. The [[Pythagoreans|Pythagorean]] philosopher [[Philolaus]] postulated that at the center of the Universe was a "central fire" around which the [[Earth]], [[Sun]], [[Moon]] and [[Planets]] revolved in uniform circular motion.<ref>Boyer, C. ''A History of Mathematics.'' Wiley, p. 54.</ref>
The [[Greek astronomy|Greek astronomer]] [[Aristarchus of Samos]] was the first known individual to propose a [[Heliocentrism|heliocentric]] model of the universe. Though the original text has been lost, a reference in Archimedes' book The Sand Reckoner describes Aristarchus' heliocentric theory. [[Archimedes]] wrote: (translated into English)
<blockquote>
You King Gelon are aware the 'Universe' is the name given by most astronomers to the sphere the center of which is the center of the Earth, while its radius is equal to the straight line between the center of the Sun and the center of the Earth. This is the common account as you have heard from astronomers. But Aristarchus has brought out a book consisting of certain hypotheses, wherein it appears, as a consequence of the assumptions made, that the universe is many times greater than the 'Universe' just mentioned. His hypotheses are that the fixed stars and the Sun remain unmoved, that the Earth revolves about the Sun on the circumference of a circle, the Sun lying in the middle of the orbit, and that the sphere of fixed stars, situated about the same center as the Sun, is so great that the circle in which he supposes the Earth to revolve bears such a proportion to the distance of the fixed stars as the center of the sphere bears to its surface.
</blockquote>
Aristarchus thus believed the stars to be very far away, and saw this as the reason why there was no visible parallax, that is, an observed movement of the stars relative to each other as the Earth moved around the Sun. The stars are in fact much farther away than the distance that was generally assumed in ancient times, which is why stellar parallax is only detectable with telescopes. The geocentric model, consistent with planetary parallax, was assumed to be an explanation for the unobservability of the parallel phenomenon, stellar parallax. The rejection of the heliocentric view was apparently quite strong, as the following passage from Plutarch suggests (On the Apparent Face in the Orb of the Moon):
<blockquote>
[[Cleanthes]] [a contemporary of Aristarchus and head of the Stoics] thought it was the duty of the Greeks to indict Aristarchus of Samos on the charge of impiety for putting in motion the Hearth of the universe [i.e. the earth], . . . supposing the heaven to remain at rest and the earth to revolve in an oblique circle, while it rotates, at the same time, about its own axis. [1]
</blockquote>
The only other astronomer from antiquity known by name who supported Aristarchus' heliocentric model was [[Seleucus of Seleucia]], a [[Hellenization|Hellenized]] [[Babylonia]]n astronomer who lived a century after Aristarchus.<ref>[[Otto E. Neugebauer]] (1945). "The History of Ancient Astronomy Problems and Methods", ''Journal of Near Eastern Studies'' '''4''' (1), p. 1–38.
{{quote|"the [[Chaldaea]]n Seleucus from Seleucia"}}</ref><ref>[[George Sarton]] (1955). "Chaldaean Astronomy of the Last Three Centuries B. C.", ''Journal of the American Oriental Society'' '''75''' (3), pp. 166–173 [169]:
{{quote|"the heliocentrical astronomy invented by Aristarchos of Samos and still defended a century later by Seleucos the [[Babylonia]]n"}}</ref><ref>William P. D. Wightman (1951, 1953), ''The Growth of Scientific Ideas'', Yale University Press p.38, where Wightman calls him [[Seleukos]] the [[Chaldea]]n.</ref> According to [[Plutarch]], Seleucus was the first to prove the heliocentric system through [[reasoning]], but it is not known what arguments he used. Seleucus' arguments for a heliocentric theory were probably related to the phenomenon of [[tide]]s.<ref>[[Lucio Russo]], ''Flussi e riflussi'', Feltrinelli, Milano, 2003, ISBN 88-07-10349-4.</ref> According to [[Strabo]] (1.1.9), Seleucus was the first to state that the [[tide]]s are due to the attraction of the Moon, and that the height of the tides depends on the Moon's position relative to the Sun.<ref>[[Bartel Leendert van der Waerden]] (1987), "The Heliocentric System in Greek, Persian and Hindu Astronomy", ''Annals of the New York Academy of Sciences'' '''500''' (1): 525–545 [527]</ref> Alternatively, he may have proved the heliocentric theory by determining the constants of a [[Geometry|geometric]] model for the heliocentric theory and by developing methods to compute planetary positions using this model, like what [[Nicolaus Copernicus]] later did in the 16th century.<ref>[[Bartel Leendert van der Waerden]] (1987), "The Heliocentric System in Greek, Persian and Hindu Astronomy", ''Annals of the New York Academy of Sciences'' '''500''' (1): 525–545 [527–9]</ref> During the [[Middle Ages]], heliocentric models may have also been proposed by the [[Indian astronomy|Indian astronomer]], [[Aryabhata]],<ref>[[Bartel Leendert van der Waerden]] (1987). "The Heliocentric System in Greek, Persian and Hindu Astronomy", ''Annals of the New York Academy of Sciences'' '''500''' (1): 525–545 [529–34]</ref> and by the [[Islamic astronomy|Persian astronomers]], [[Ja'far ibn Muhammad Abu Ma'shar al-Balkhi|Albumasar]]<ref>[[Bartel Leendert van der Waerden]] (1987). "The Heliocentric System in Greek, Persian and Hindu Astronomy", ''Annals of the New York Academy of Sciences'' '''500''' (1): 525–545 [534–7]</ref> and [[Al-Sijzi]].<ref name=Nasr>{{Cite book |last=Nasr |first=Seyyed H. |authorlink=Hossein Nasr |date=1st edition in 1964, 2nd edition in 1993 |title=An Introduction to Islamic Cosmological Doctrines |edition=2nd |publisher=1st edition by [[Harvard University Press]], 2nd edition by [[State University of New York Press]] |isbn=0791415155 |pages=135–6}}</ref>
[[Image:ThomasDiggesmap.JPG|thumb|left|Model of the [[Copernicus|Copernican]] universe by [[Thomas Digges]] in 1576, with the amendment that the stars are no longer confined to a sphere, but spread uniformly throughout the space surrounding the [[planet]]s.]]
The Aristotelian model was accepted in the [[Western world]] for roughly two millennia, until [[Copernicus]] revived Aristarchus' theory that the astronomical data could be explained more plausibly if the [[earth]] rotated on its axis and if the [[sun]] were placed at the center of the Universe.
{{cquote|In the center rests the sun. For who would place this lamp of a very beautiful temple in another or better place than this wherefrom it can illuminate everything at the same time?|20px|20px|[[Copernicus]]| in Chapter 10, Book 1 of ''De Revolutionibus Orbium Coelestrum'' (1543)}}
As noted by Copernicus himself, the suggestion that the [[Earth's rotation|Earth rotates]] was very old, dating at least to [[Philolaus]] (c. 450 BC), [[Heraclides Ponticus]] (c. 350 BC) and [[Ecphantus the Pythagorean]]. Roughly a century before Copernicus, Christian scholar [[Nicholas of Cusa]] also proposed that the Earth rotates on its axis in his book, ''On Learned Ignorance'' (1440).<ref>Misner, Thorne and Wheeler (1973), p. 754.</ref> Aryabhata (476–550), [[Brahmagupta]] (598–668), [[Albumasar]] and [[Al-Sijzi]], also proposed that the Earth rotates on its axis.{{Citation needed|date=April 2010}} The first [[Empirical research|empirical evidence]] for the Earth's rotation on its axis, using the phenomenon of [[comet]]s, was given by [[Nasīr al-Dīn al-Tūsī|Tusi]] (1201–1274) and [[Ali Kuşçu]] (1403–1474).{{Citation needed|date=April 2010}} Tusi, however, continued to support the Aristotelian universe, thus Kuşçu was the first to refute the Aristotelian notion of a stationary Earth on an [[empirical]] basis, similar to how Copernicus later justified the Earth's rotation. [[Al-Birjandi]] (d. 1528) further developed a theory of "circular [[inertia]]" to explain the Earth's rotation, similar to how [[Galileo Galilei]] explained it.<ref>{{Cite journal |last=Ragep |first=F. Jamil |year=2001a |title=Tusi and Copernicus: The Earth's Motion in Context |journal=Science in Context |volume=14 |issue=1–2 |pages=145–63 |publisher=[[Cambridge University Press]] }}</ref><ref>{{Cite journal |last=Ragep |first=F. Jamil |year=2001b |title=Freeing Astronomy from Philosophy: An Aspect of Islamic Influence on Science |journal=Osiris, 2nd Series |volume=16 |issue=Science in Theistic Contexts: Cognitive Dimensions |pages=49–64 & 66–71}}</ref>
[[Image:Libr0309.jpg|thumb|[[Johannes Kepler]] published the ''[[Rudolphine Tables]]'' containing a star catalog and planetary tables using [[Tycho Brahe]]'s measurements.]]
Copernicus' [[Heliocentrism|heliocentric model]] allowed the stars to be placed uniformly through the (infinite) space surrounding the planets, as first proposed by [[Thomas Digges]] in his ''Perfit Description of the Caelestiall Orbes according to the most aunciente doctrine of the Pythagoreans, latelye revived by Copernicus and by Geometricall Demonstrations approved'' (1576).<ref name = "Misner-p755">Misner, Thorne, and Wheeler (1973), p.755.</ref> [[Giordano Bruno]] accepted the idea that space was infinite and filled with solar systems similar to our own; for the publication of this view, he was [[execution by burning|burned at the stake]] in the [[Campo de' Fiori|Campo dei Fiori]] in Rome on 17 February 1600.<ref name = "Misner-p755"/>
This cosmology was accepted provisionally by [[Isaac Newton]], [[Christiaan Huygens]] and later scientists,<ref name = "Misner-p755">Misner, Thorne, and Wheeler (1973), p. 755–756.</ref> although it had several paradoxes that were resolved only with the development of [[general relativity]]. The first of these was that it assumed that space and time were infinite, and that the stars in the universe had been burning forever; however, since stars are constantly radiating [[energy]], a finite star seems inconsistent with the radiation of infinite energy. Secondly, Edmund Halley (1720)<ref>Misner, Thorne, and Wheeler (1973), p. 756.</ref> and [[Jean-Philippe de Cheseaux]] (1744)<ref>{{cite book | author = [[Jean-Philippe de Cheseaux|de Cheseaux JPL]] | year = 1744 | title = Traité de la Comète | publisher = Lausanne | pages = 223ff}}. Reprinted as Appendix II in {{cite book | author = Dickson FP | year = 1969 | title = The Bowl of Night: The Physical Universe and Scientific Thought | publisher = M.I.T. Press | ___location = Cambridge, MA | isbn = 978-0262540032}}</ref> noted independently that the assumption of an infinite space filled uniformly with stars would lead to the prediction that the nighttime sky would be as bright as the sun itself; this became known as [[Olbers' paradox]] in the 19th century.<ref>{{cite journal | author = [[Heinrich Wilhelm Matthäus Olbers|Olbers HWM]] | year = 1826 | title = Unknown title | journal = Bode's Jahrbuch | volume = 111}}. Reprinted as Appendix I in {{cite book | author = Dickson FP | year = 1969 | title = The Bowl of Night: The Physical Universe and Scientific Thought | publisher = M.I.T. Press | ___location = Cambridge, MA | isbn = 978-0262540032}}</ref> Third, Newton himself showed that an infinite space uniformly filled with matter would cause infinite forces and instabilities causing the matter to be crushed inwards under its own gravity.<ref name = "Misner-p755"/> This instability was clarified in 1902 by the [[Jeans instability]] criterion.<ref>Jeans, J. H. (1902) ''Philosophical Transactions Royal Society of London, Series A'', '''199''', 1.</ref> One solution to these latter two paradoxes is the [[Carl Charlier|Charlier universe]], in which the matter is arranged hierarchically (systems of orbiting bodies that are themselves orbiting in a larger system, ''ad infinitum'') in a [[fractal]] way such that the universe has a negligibly small overall density; such a cosmological model had also been proposed earlier in 1761 by [[Johann Heinrich Lambert]].<ref>Rindler, p. 196; Misner, Thorne, and Wheeler (1973), p. 757.</ref> A significant astronomical advance of the 18th century was the realization by [[Thomas Wright (astronomer)|Thomas Wright]], [[Immanuel Kant]] and others that stars are not distributed uniformly throughout space; rather, they are grouped into [[galaxy|galaxies]].<ref>Misner, Thorne and Wheeler, p.756.</ref>
The modern era of [[physical cosmology]] began in 1917, when [[Albert Einstein]] first applied his general theory of relativity to model the structure and dynamics of the universe.<ref name="einstein_1917">{{cite journal | last = Einstein | first = A | authorlink = Albert Einstein | year = 1917 | title = Kosmologische Betrachtungen zur allgemeinen Relativitätstheorie | journal = Preussische Akademie der Wissenschaften, Sitzungsberichte | volume = 1917 (part 1) | pages = 142–152}}</ref> This theory and its implications will be discussed in more detail in the following section.
==Theoretical models==
[[Image:Cassini-science-br.jpg|thumb|High-precision test of general relativity by the [[Cassini-Huygens|Cassini]] space probe (artist's impression): [[radio]] signals sent between the Earth and the probe (green wave) are [[Shapiro effect|delayed]] by the warping of [[space and time]] (blue lines) due to the [[Sun]]'s mass.]]
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